1. Field of the Invention
The present invention relates particularly to an acceleration sensor and a pressure sensor of a semiconductor device for transducing displacement into electrical signals by utilizing a piezo-resistance effect of semiconductor crystal such as silicon.
2. Description of the Related Background Art
The recent development of micro-machining technology has made it possible to fabricate a semiconductor acceleration sensor by forming a thin film on a semiconductor wafer and by etching it (see IEEE Transactions on Electron Devices, Vol. ED-26, No. 12, Dec. 1979 for example).
FIG. 3 show a semiconductor acceleration sensor fabricated by means of a prior art micro-machining technology, wherein FIG. 3A is a plan view thereof and FIG. 3B is a section view. A silicon base 1 is etched to form a cantilever 2 and a weight 3. Here, the cantilever 2 is formed to be thinner than other parts by means of etching and is deformed by acceleration in the direction of an arrow in FIG. 3B. The deformation of the cantilever 2 is detected by a piezo-resistance effect of a diffused resistor 4a formed on the upper surface of the cantilever 2 and the acceleration is found by comparing with a diffused resistor 4b. Here, the diffused resistors 4a and 4b are connected to a high concentrate diffusion domain 5 and output terminals 8. Further, an upper stopper 6 and a lower stopper 7 are disposed and the whole structure is disposed on a ceramic substrate 10 in order to prevent the cantilever 2 from being destroyed.
FIG. 4A shows a semiconductor acceleration sensor fabricated by means of micro-machining disclosed in Japanese Patent Laid-Open No. Hei. 1-302167, wherein a groove 35 is formed by means of etching near a supporter 9 of the cantilever 2 to provide a thin portion 36. Diffused resistors 4c, 4d, 4e and 4f which compose a bridge circuit 50 are disposed on the upper surface of the sensor. The diffused resistors 4c and 4f function as reference resistors and are disposed on the upper part of the supporter 9. The diffused resistors 4d and 4e function as variable resistors for detecting the deformation of the thin portion 36 and are disposed orthogonally to the reference resistors. FIG. 4B shows a detection circuit of the device shown in FIG. 4A.
Although it is necessary to form the thin portion 36 shown in FIG. 4A in the prior art semiconductor acceleration sensor in order to enhance the detection sensitivity, the mechanical strength of the whole drops due to that.
While the thickness of the thin portion 36 is related to the detection sensitivity, it is necessary to strictly control the composition, temperature and agitating conditions of etching fluid and a number of manufacturing processes increase such as formation of pattern of masking in order to obtain a uniform thickness because the thin portion 36 is formed by means of etching.
Further, because the diffused resistors 4a and 4b are disposed on the upper surface of the acceleration sensor and the weight 3 is formed as shown in FIG. 3A, an area of the upper surface of the acceleration sensor becomes large. Therefore, a number of sensors taken out of one silicon substrate is limited and it has been difficult to lower the production cost for example. Further, a predetermined value has to be kept for a width of the cantilever 2, i.e. a width in the depth direction from the front side of the figure, in order to keep a certain strength of the thin portion 36 also in the acceleration sensor in FIG. 4A. Accordingly, the area of the upper surface of the sensor cannot be reduced similarly to the acceleration sensor in FIG. 3A. Therefore, a number of sensors taken out of a semiconductor wafer is also limited and it is difficult to lower the cost.
Still more, because the diffused resistors 4 for detecting the acceleration are placed on the surface of the acceleration sensor which receives the acceleration, it has been necessary to dispose the diffused resistors 4 such that a difference of values of resistance of the reference resistors disposed on the supporter 9 and the variable resistors becomes large.
Accordingly, the present invention has been devised to solve the aforementioned problems and has been intended to manufacture a large number of sensors readily from one semiconductor wafer to obtain low cost semiconductor acceleration sensors and pressure sensors.